The invention relates to an apparatus for fixing the composition of the gas content of internal combustion engine cylinders. A system is known in which the air ratio .lambda. pertaining to the aspirated fresh air and optimal for normal conditions is set in accordance with load and rpm by means of the open or closed loop controlled adjustment of an exhaust feedback valve and is governed with the aid of air quantity measurement (German Offenlegungsschrift No. 24 09 774). However, in this apparatus, a quantity of exhaust gas, not clearly definable, mixes in with the aspirated fresh-air flow, the extent depending on pressure and temperature of the fresh air and the exhaust and also on the state of the engine and, if it is present, the supercharger. Large deviations from the normal state result, particularly at extremes of ambient temperature and because of differing ambient pressure relating to elevation above sea level. It is also known that the creation of toxic substances during combustion in the cylinder is substantially determined by the flame velocity and the peak combustion temperature. These variables are dependent, in turn, on the total cylinder content, among other factors. In the known system, these variables cannot be sufficiently attended to and/or controlled.